Recently, a new device family has come into being, namely, thermo-optically tunable, thin-film filters. These devices, which are made from amorphous semiconductor materials, exploit what had previously been viewed as an undesirable property of amorphous silicon, namely, its large thermo-optic coefficient. The performance of these devices is based on trying to maximize thermo-optic tunability in thin-film interference structures, instead of trying to minimize it as is often the objective for conventional fixed filters.
FIG. 1 shows the basic device structure for the thermo-optically tunable thin film filter. The particular structure illustrated is a single cavity Fabry-Perot type filter 10. It includes a heater film 12 integrated into the optical interference design, and a Fabry-Perot cavity made of a pair of thin film mirrors 14(a) and 14(b) separated by a spacer cavity 16. In this example, heater film 12 is made of ZnO or polysilicon, so it is both electrically conductive and optically transparent at 1500 nm. Thin film mirrors 14(a) and 14(b) are alternating quarter wave pairs of high and low index films. The two materials are a-Si:H (n=3.67) and non-stoichiometric SiNx (n=1.77). Because of the large index contrast between a-Si and SiNx, a relatively small number of mirror pairs is required. Even 4 pairs yields reflectivity R=98.5% at the design wavelength, and 5 pairs yields R=99.6%. Cavity 16 is an integral number of half-waves, typically two to four, of amorphous silicon.
The amorphous thin films can be deposited by various physical vapor deposition techniques such as sputtering, or chemical vapor deposition techniques including plasma-enhanced enhanced chemical vapor deposition (PECVD). PECVD is a particularly flexible and homogeneous thin film process, and control of the basic deposition parameters such as plasma power, total gas pressure, hydrogen partial pressure, gas ratios, flow rates, and substrate temperature can be used to significantly modify film density and stoichiometry which in turn influence index, optical absorptivity, and thermo-optic coefficients. In addition, hydrogenation of the a-Si films can be used to quench dangling bonds and thereby decrease defect densities which, in turn, reduces infrared absorptivity. As a plasma based technique, PECVD offers the process variability needed to more easily produce dense, compliant films of several optically distinct but process-compatible materials, such as amorphous silicon and amorphous silicon nitride, with widely different indices. Transitions between materials can be accomplished by controlling gas mixtures, without breaking vacuum.
The finesse that is achievable with the thermo-optically tunable, thin film filters is illustrated by FIG. 2. In this case, the filter was a single cavity configuration using 6 mirror cycles and a fourth order spacer (4 half waves). The −3 dB width was 0.085 nm for a free spectral range of 388 nm and a finesse of approximately F=4,500.
The thermal tuning that is achievable is illustrated by FIG. 3. The configuration used an amorphous silicon spacer with dielectric mirrors (tantalum pentoxide high index and silicon dioxide low index layers, deposited by ion-assisted sputtering, R=98.5% mirror reflectivity). That structure was heated in an oven from 25 C. to 229 C. The tuning was approximately 15 nm or dλ/dT=0.08 nm/K.
Finally, the benefit of constructing a tunable filter with all-PECVD films using amorphous silicon not only for the spacer but also for the mirror high index layers is illustrated in FIG. 4. This filter, with 4 period mirrors, incorporated an electrically conductive ZnO layer for heating internal to the film stack, which is able to achieve much higher local film temperatures than if it the heater was separate from the film stack. The tuning range in this example was 37 nm.
Further details about these new structures can be found in U.S. patent application Ser. No. 10/174,503 filed Jun. 17, 2002, entitled “Index Tunable Thin Film Interference Coatings;” and U.S. patent application Ser. No. 10/211,970 filed Aug. 2, 2002, entitled “Tunable Optical Instruments,” both of which are incorporated herein by reference.